Columbium-base alloy of improved fabricability



United States Patent 3,236,638 CGLUMBHIM-BASE ALLOY 0F IMPRUVED FABRHCABILITY lack W. Clark, Milford, Ohio, assignor to General Electric Company, a corporation of New York No Drawing. Filed Nov. 1, 1963, Ser. No. 320,919 The portion of the term of the patent subsequent to Oct. 2, 1979, has been disclairned 9 Claims. (Cl. 75-174) This invention relates to columbium-base alloys, particularly those of improved fa'bricability.

This application is a continuation-in-part of application Ser. No. 117,795-Clark, filed June 19, 1961, now abandoned, and assigned to the assignee of the present application, and which is, in turn, a continuation-in-part of application Ser. No. 73,051Clark, filed December 1, 1960, now Patent 3,056,672, issued October 2, 1962, and assigned to the assignee of this application.

Although previous alloy studies relating to the element columbium have resulted in columbium-base alloys of improved oxidation resistance, improved strength and somewhat improved fabricability, further efforts have been toward the development of an alloy which approaches unalloyed columbium in ease of processing and fabrication but at higher levels of strength. Although very high strengths are required for some applications of columbium-base alloys, there is currently a need for highly fabricatable, moderate strength columbium-base alloys for use in intricate structures such as for alkali metal containment in power systems. Presently the fabrication of such structures is confined to fabricatable, but low strength, alloys such as the Cb-lZr type.

Therefore it is an object of this invention to provide a columbium-base alloy having improved fabricability characteristics and aswelded ductility along with moderate strength at high temperatures.

Another object is to provide a moderate strength columbium-base alloy having improved fabricability through critical control of interstitial elements such as oxygen, carbon and nitrogen.

A further object is to provide a columbium-base alloy, having the properties stated in the above objects, which can be produced by less rigorous techniques than are required for comparable alloys previously known in the art.

These and other objects and advantages will become apparent from the following detailed description.

In one aspect, this invention provides an improved columbium-base alloy consisting essentially of, approximately and by weight: 25% tungsten, 0.53% of at least one metal selected from the group zirconium and hafnium, 0.042.0% yttrium, carbon from 0.01 to 0.2% and in an amount such that the zirconium or hafnium to carbon atomic ratio is in the range of from about 0.5 :1 to 3.021, and the balance columbium and incidental impurities. The preferred range of tungsten content is about 510%, the first preferred range of yttrium content is about 0.08l.5%. It is alo preferred that oxygen be present in the range of 0.01-0.04%. In further optimizing the alloys, the second preferred maximum retained yttrium content is 0.75%.

It appears that the deformation characteristics of columbium alloys are strongly dependent on the absolute "ice level of the interstitial elements and the form in which they are present. This is particularly true in the case of fusion weldments which can be embrittled both by the supersaturated interstitial solutions formed in the Weld and by certain aging reactions which occur during extended exposure under service conditions. Many of the dilficulties which arise in sheet rolling can be attributed to inadequate control of, or protection from, the interstitial elements.

Corrosion by liquid alkali metals can be rapidly accelerated by enrichment of grain boundary regions with uncombined oxygen or carbon. However, significant strengthening can be achieved by judicious additions of these interstitial elements along with stabilizing elements such as zirconium and hafnium. As has been shown in the above mentioned Patent 3,056,672 of which this is a continuation-in-part, the formation of stable carbide dispersions within a particular range has been shown to substantially increase the creep strength of columbium. The present invention combines such beneficial effects of zirconium and hafnium in promoting the formation of stable carbides or carbonitrides for strengthening, with the control of oxygen by critical additions of yttrium for improved fabricability.

The element yttrium has a particularly unique combination of properties which appear to have contributed to its usefulness as a gettering addition in the arc-melting of columbium base alloys. Because it has the highest melting point of the highly reactive Group III A and rare earth elements, it permits solid-state sintering of consumable eleotrode-s. Yttrium has a relatively low vapor pressure at the melting point of columbium so that it is retained in the columbium alloy melt. Its highly volatile sub-oxide, YO, promotes purification by vaporization. The free energy of formation of the equilibrium oxide, Y O is the highest known at temperatures above 4000 F. (94.8 kcal./g.-atom 0 The thermodynamic stability of Y O combined with the immiscibility between liquid columbium and liquid yttrium provides further purification through slag formation.

Although the elements cerium, lanthanum, and praseodymium might be expected to act in a manner related to that of yttrium, their lower melting points would make sintering more difiicult and their higher densities would slow separation from the columbium-rich liquid. I presently consider that the cumulative effect of the various less desirable properties caused by the addition of rare earth metals other than yttrium would be sufficient to result in alloys neither possessing the essential advantages of nor being within the present invention.

I have discovered that by adding a critical minimum amount and less than a critical maximum amount of yttrium to the ingredients of a melt of the type containing 0.5-3.0 weight percent zirconium or hafnium, 5-25 weight percent tungsten, carbon in an amount of from 0.01 to approximately 0.2 weight percent such that the zirconium or hafnium to carbon atomic ratio is about 0.5-3.0 and the balance columbium, an alloy of greatly improved fabricability could be obtained.

In addition to greatly benefitting the properties, particularly Weldability and resistance to liquid alkali metals, it has been discovered that the present invention allows the production of such improved alloys by techniques and in apparatus which need not meet the rigorous and meticulous demands required for the casting of ingots of many prior commercial refractory metal alloys.

The following Table I includes typical examples of alloys melted in the study of the present invention. The elements are presented in weight percent additions to the melt. The carbon shown is that which was added to the melt exclusive of and in addition to 0075-0081 weight percent carbon included in the columbium powder used. The designation and 0 represents the oxygen content of the powder and of the melted ingot respectively, and C is the carbon content of the ingot.

TABLE I Alloying additions to Cb (weight percent) carbon and oxygen. It is to be noted that, in the presence of 1 weight percent zirconium, small tungsten additions appear to be superior to either molybdenum or vanadium in respect to fabricability characteristics.

It is to be noted that the alloys 55A, 55C, 72, 81 and 83 including yttrium additions of between 0.52 weight percent to tie up the oxygen, resulted in alloys which were readily cold-rollable and, in general, had good weld ductility Whereas as the other alloys in Table I, including the higher carbon alloys 71 and 73 were not so readily fabricative. In connection with alloys 71 and 73, however, it is to be noted that 0.1 weight percent carbon was added to The alloys represented by Table I were prepared as coldpressed and sintered electrodes which were consumably arc-melted under partial pressure of helium as 1.2 inch diameter, 400-500 gram ingots. After conditioning to remove surface irregularities, the ingots were vacuum annealed at 3500 F. for 1 hour to improve homogeneity. Ingots were then extruded to 0.6 inch diameter at 2600 2700 F. and swaged to 0.4 inch diameter at 20002200 F. After grinding to remove any contaminated layer, to swaged bar was rolled to 0.040 inch strip. Alloys which could not be rolled at room temperature were successfully worked at 800-1200 F., a temperature range considered in the art to be a cold-working rather than a hot-working range. Specimens were are welded in a chamber filled with high purity helium. The following Table II shows the percentage reduction through coldrolling and the weld ductility of the alloys of Table I:

2 Full bend.

The addition of zirconium improves fabricability, probably through removal of interstitial elements such as a nitrogen from solid solution in addition to its effects on the other ingredients of the melt to result 1n a total of about 0.18 weight percent carbon. Therefore, although these alloys were found to be ductile after heat treatment, it is preferred that the carbon content be maintained at less than about 0.18 Weight percent Within the zirconium or hafnium to carbon ratio of the present invention inalloys of the specific types of alloys 71 and 73. In alloys of the invention in general, I consider that the carbon content can be as high as approximately 0.2 weight percent or even higher. It is to be understood that this approximation includes alloys with somewhat higher carbon contents so long as they come within the invention.

In studying the structure of the alloy ingots and the surface slag obtained during melting, it was noted through X- ray diffraction that the surface slag is essentially all cubic Y O a highly stable oxide. Furthermore, small internal spheroids of Y O remain virtually unchanged upon exposure to temperatures of at least 3500 F., substantiating the stability of this phase in a columbium-rich matrix. These oxide particles are quite effective in inhibiting grain growth during annealing of fusion welded metal.

Some 25 alloys with tungsten contents of 5-10 weight percent and both yttrium and oxygen contents within the first preferred ranges of 0.04 to 1.5 weight percent yttrium and 0.01 to 0.04 weight percent oxygen have been melted as ingots weighing from less than one to about thirty pounds. Each of these alloys was melted satisfactorily, had good fabricability, exhibited ductile fusion weldments, and had adequate grain-refinement characteristics. Alloys outside the preferred ranges but still within the invention had less attractive but still useful characteristics.

Specific examples which document these observations are shown in the following Table III wherein the nominal composition is the composition of the electrode before melting. Each alloy was processed as described above to .040.060 inch sheet. Fusion beads, with full penetration of the sheet, were produced by the tungsten-arc, inert gas (TIG) welding process. The weld ductility was tested at F. on specimens bent over a radius of 2 times the sheet thickness. Grain sizes were measured by standard metallographic techniques.

TABLE III Analysis Grain Size (mm) Nominal Composition 100 F. Weld Alloy (wt. percent) Ductility Comments Y C O Annealed Welded Cb-fiW-lZr-IY.-. l7 07 Cb-5W-lZr-1 Y. 08 02 05 Ob-5W-1Zr-1Y- 04 13 Cb-5W-1Zr-1Y- 15 03 09 Cb-5W-lZr-1Y 34 04 10 Cb-5W-1Zr-1Y (low 73 11 Clo-W-1Zr-1Y (low 0, O 24 .15 .5 Cb-5W-1Zr-1Y 035 15 4 Cb-5W-1Zr-1Y- 02 Cb-5W-1Zr 01 Cb5W-1Zr (high purity)- 01 35 95 Also veg) weak ow Cb-5W-1Zr (low 0;) 01 13 4 Cb-W-1Zr-1Y 15 Cb-lOW-1Zr-2Y-J C, *2. 0 Slag entrapped. Cb-10W-1Zr-2Y-.10 *2. 0 Could not melt.

Cb-5W-lZr-L5Y *1. 5 Melt OK.

*Nominal analysis. Retained analysis not made.

Fusion weldments in the alloys containing yttrium and oxygen within the first preferred range are completely ductile in the as-welded condition. Although postweld anneals such as at 2500 F. can restore ductility to a number of welded alloys, alloys of the present invention do not require such treatment. As a further example of fabricability as indicated in Table II above, the as-welded alloys 55A and 55C can be cold rolled at room temperature to .001 inch thick foil after full recrystallization (2500 F.) of the welded strip.

Most of the oxygen in the starting material reacts with the yttrium and is removed as YO vapor and as a slag rich in Y O Referring in particular to the oxygen contents of Tables I and III and the amounts of yttrium retained by alloys within the first preferred range for fabricability improvement, it can be seen that preferably no more than a maximum of about 0.04 weight percent oxygen should be in the alloy ingot. Since it is desirable to retain some oxygen in the solidified ingot for grainrefinement purposes in combination with yttrium, generally as Y O the preferred minimum oxygen content is about 0.01 weight percent. Oxygen can be considered to be an incidental impurity in the generic description of alloys of the invention, but it is preferably specified to be within the above-stated limits. In order to reduce the oxygen content to below the 0.04 weight percent preferred maximum, a minimum of approximately 12 parts by weight of yttrium are required per part by weight of oxygen in the starting material in excess of the preferred maximum to be obtained in the melted ingot.

At retained yttrium levels below .04%, the alloys are either brittle at -100 F. in the as-welded condition (with high oxygen contents) or (with low oxygen contents) have a very large grain size in both the welded condition and after annealing at temperatures above about 3200 F. Grain refinement may be very important with respect to applications such as liquid metal containment in space power systems. Compared to bulk corrosion rates of columbium alloys, the grain boundaries are much more rapidly attacked by the liquid alkali metals (lithium, sodium, potassium) used as working fluids in such systems. Therefore, large-grained structures are particularly sensitive to alkali metal corrosion or weeping.

Alloys which contain greater than about 1.5% yttrium are ditficult to melt. Specifically, an alloy which contained 2.0% yttrium in the arc-melting electrode was prepared in two different heats (designated 83 and 83B). The first melted slowly at the maximum power output of the furnace employed, but there was excessive entrapment of a slag rich in Y O in the central portion of the ingots. The second heat could not be consumably arc melted at maximum power due to equipment limitations. However, I consider that, with suflicient equipment capacity, alloys of the invention containing up to 2.0 weight percent yttrium can be satisfactorily melted. The melting difliculties are probably associated with the low thermionic work function of yttrium.

In a stress rupture test conducted at 20,000 p.s.i. and 2000" F. in high vacuum, alloy 55A was compared with alloy 54 and a currently available, readily fabricatable alloy of 1% zirconium, balance columbium, designated as alloy A, which can be cold-rolled to a reduction and is also ductile in the aswelded condition. The following Table IV shows the vastly superior strength quality in alloy 55A, combined with weld ductility equivalent to that of the Weaker alloy A.

TABLE IV Stress Rupture Life Bend Ductility As- Alloy At 20,000 p.s.i. and welded (Bend 2,000 F. (in hours) angle), degrees 52v 5 10 55A 81.6 A 0.5 105 Based on the stress for rupture in 10 hours, alloy 55A shows a superiority of at least 200 F. over the Cb-1Zr alloy designated as alloy A.

The addition of critical amounts of yttrium to tungstenand-zirconium-bearing columbium base alloys in which the zirconium to carbon atomic ratio is from 0.5 to 3.0 produces an alloy system with improved fabricability, while maintaining a moderate strength, significantly higher than that of columbium base alloys of comparable fabricability. The applicability of these alloys for liquid metal containment appears to be particularly attractive. The optimum form of the alloy is identified as alloy 55A and contains, approximately by weight, 5% tungsten, 1% zirconium, 0.08% carbon, 0.010.04% oxygen, and a retained amount of about 0.170.25% yttrium. This alloy has been readily cold-rolled and has resulted in fusion weldments which are ductile in the as-welded condition. Its fabricability is at least equivalent to that of the best commercially available arc-cast columbium base alloys, and its mechanical properties indicate a superiority of ZOO-300 F. over such as Cb-lZr alloys. The addition of yttrium as shown increases the tolerance for interstitiais in the starting material for arc-melting and allows less expensive, less rigid control of the purity of the environment during melting due to the volatility of Y0 and the formation of a highly reactive, immiscible slag.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A columbium-base alloy consisting essentially of, by weight: about 525% tungsten; about 05-30% of at least one metal selected from the group consisting of zirconium and hafnium; about 0.042.0% yttrium; carbon in an amount from about 0.01% to approximately 0.2% and such that the atomic ratio of metal selected from said group to carbon is from about 0.5 :1 to about 3.0:1; with the balance columbium and incidental impurities.

2. A columbium-base alloy consisting essentially of, by weight: about 510% tungsten; about 0.53.0% of at least one metal selected from the group consisting of zirconium and hafnium; about 0.042.0% Yttrium; carbon in an amount from about 0.01% to approximately 0.2% and such that the atomic ratio of metal selected from said group to carbon is from about 0.5 :1 to about 3.0:1; with the balance columbium and incidental impurities.

3. A columbium-base alloy consisting essentially of, by weight: about 510% tungsten; about 05-30% of at least one metal selected 'from the group consisting of zirconium and hafnium; about 0.08l.5% yttrium; carbon in an amount from about 0.01% to approximately 0.2% and such that the atomic ratio of metal selected from said group to carbon is from about 0.5 :1 to about 3.011; with the balance columbium and incidental impurities.

4. A columbium-base alloy consisting essentially of, by weight: about 510% tungsten; about 0.53.0% of at least one metal selected from the group consisting of zirconium and hafnium; about 0.080.75% yttrium; carbon in an amount from about 0.01 to about 0.18% and such that the atomic ratio of metal selected from said group to carbon is from about 0.521 to 30:1; with the balance columbium and incidental impurities.

5. A columbium-base alloy consisting essentially of, by weight: about 5-10% tungsten; about 0.5-3.0% of at least one metal selected from the group consisting of zirconium and hafnium; about 0.042.0% yttrium; carbon in an amount from about 0.01% to approximately 0.2%, said carbon being in an amount such that the atomic ratio of metal selected from said group to carbon is from about 0.5 :1 to about 3.0:1; about 0.01-0.04% oxygen; with the balance columbium and incidental impurities.

6. A columbium-base alloy consisting essentially of, by weight: about 510% tungsten; about 05-30% of at least one metal selected from the group consisting of zirconium and hafnium; 0.08-1.5% yttrium; about 0.01- 0.18% carbon, said carbon being in an amount such that the atomic ratio of metal selected from said group to carbon is from about 0.521 to about 3.0: 1; about 0.01- 0.04% oxygen; with the balance columbium and incidental impurities.

7. A columbium-base alloy consisting essentially of, by weight: about 510% tungsten, about 1% zirconium, about 0.080.75% yttrium, from about 0.01 to about 0.18% carbon with the atomic ratio of zirconium to carbon being from about 0.5:1 to about 3.0:1, about 0.0l 0.04% oxygen, with the balance columbium and incidental impurities.

8. A columbium-base alloy consisting essentially of about, by weight: 5% tungsten, 1% zirconium, 0.08- 0.75% yttrium, 0.08% carbon, 0.010.04% oxygen, with the balance columbium and incidental impurities.

9. A columbium-base alloy consisting essentially of about, by weight: 5% tungsten, 1% zirconium, 0.17- 0.25% yttrium, 0.08% carbon, 0.010.04% oxygen, with the balance columbium and incidental impurities.

References Cited by the Examiner UNITED STATES PATENTS 2,973,261 2/1961 Frank 174 3,028,236 4/1962 Wlodek 75174 3,056,672 10/1962 Clark 75174 3,156,560 11/1964 Semmel 75--174 FOREIGN PATENTS 323,315 6/1932 Canada. 821,796 10/1959 Great Britain.

OTHER REFERENCES I. W. Semmel, Ir., The Effect of Rare-Earth Metal Additions on the Ductility of Arc-Melted Group Va Metals, Report No. 57-RL-1736, April 1957, published by Research Information Section, The Knolls, Schenectady. New York, 12 pp.

Rare Metals Handbook, C. A. Hampel, 1954, Reinhold Publishing Corp., New York, page 329.

The Condensed Chemical Dictionary, Sixth Edition, Reinhold Publishing Corp., New York, 1961, page 1239.

DAVID L. RECK, Primary Examiner.

WINSTON A. DOUGLAS, Examiner.

W. C. TOWNSEND, C. N. LOVELL,

Assistant Examiners. 

1. A COLUMBIUM-BASE ALLOY CONSISTING ESSENTIALLY OF, BY WEIGHT: ABOUT 5-25% TUNGSTEN; ABOUT 0.5-3.0% OF AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF ZIRCONIUM AND HAFNIUM; ABOUT 0.04-2.0% YTTRIUM; CARBON IN AN AMOUNT FROM ABOUT 0.01% TO APPROXIMATELY 0.2% AND SUCH THAT THE ATOMIC RATIO OF METAL SELECTED FROM THE GROUP TO CARBON IS FROM ABOUT 0.5:1 TO ABOUT 3.1:1; WITH THE BALANCE COLUMBIUM AND INCIDENTAL IMPURITIES. 